AND ITS PRODUCTS
During the dry distillation of coal, as in the manufacture of illuminating
gas and in the preparation of coke, crude tar is coUected as a secondary
product. When subjected to further treatment this gives, on distillation,
tar oils, these being distinguished according to the temperature at which
they are collected, as light, medium, heavy and anthracene oils. The residue
from the distillation is pitch. From the tight oils are obtained, by further
distillations, benzene (benzole) and the toluenes (toluoles), which are used in
the dye and explosive industries and as solvents. The medium, heavy
and anthracene oils yield other products of industrial importance, such as
naphthalene, anthracene, carbolic acid, pyridine bases and impregnating oils.
All of these products are considered separately in succeeding paragraphs,
the tests commonly made in each case being indicated.
Sampling.—With very viscous liquids, such as crude tar and heavy
and anthracene oils, it is not easy to obtain a sample exactly representing
the mean composition of the whole mass. To take such a sample from the
vessel or tank containing the material, use is made of a metallic dipping
cylinder 4-5 cm. wide and closed at the bottom by a plug which is raised
or lowered by means of an iron wire passing through the cylinder itself.
The cylinder is filled several times and all the samples mixed so as to obtain
as representative a sample as possible. With a non-viscous liquid, however,
it is sufficient to mix the mass before extracting the sample.
With solid products such as naphthalene and anthracene, the mass
must be examined to see if it is all of the same appearance, and samples
should be drawn from different parts and mixed before analysis.
This is a dense, black, oily liquid with a characteristic odour due to the
presence of aromatic hydrocarbons, phenols, naphthalene and pyridine
bases. When it is to be distilled, the tests made are 1-4 (below), but if it is
to be used as a fuel, the ash content and the calorific power are determined
as with mineral oils (q.v.).
1. Determination of the Water.—Since tar exhibits a tendency to
allow the water present to separate, either the sample should be taken
immediately after the whole mass has been mixed, or the separated water